The term “liquefaction plant” is understood to mean any plant or any process that is used for cooling and liquefying a gaseous medium, for example, hydrogen, a noble gas such as helium, neon, etc., oxygen, nitrogen, or a hydrocarbon mixture, in particular natural gas.
In known arrangements, there is provided at least one liquefaction plant for liquefying a gaseous medium and at least one storage tank which is used to store the liquefied medium. Generally, two transfer lines are provided which connect the liquefaction plant and the storage tank. A first transfer line is used to transfer the liquefied medium from the liquefaction plant to the storage tank, and a second transfer line (a return line) is used to transfer the flash and boil-off gas of the liquefied medium from the storage tank to the liquefaction plant. It is usual to provide a mechanism, such as a shut-off valve, in each transfer line which can be activate to stop the flow in the transfer line.
Usually, the transfer lines are vacuum insulated lines. By providing two, distinct and physically separated transfer lines between liquefaction plant and storage tank, it is possible at the same time to transfer liquefied medium from the liquefaction plant into the storage tank and gaseous medium from the storage tank into the liquefaction plant.
A typical known arrangement is shown in FIG. 1. A liquefaction plant V represented simply as a black box is connected to a represented storage tank S having an outer tank 1 and an inner tank 2. The arrangement includes a first transfer line 3, in which a shut-off valve a is arranged. The medium liquefied in the liquefaction plant V is supplied to the storage tank S through line 3. A second transfer line 4 is provided, in which a shut-off valve b is arranged, is provided. Gaseous medium is transferred from the storage tank S into the liquefaction plant V through second transfer line 4 (which can also be referred to as a return line). The outlet of transfer line 3 and the inlet of the return transfer line 4 are located in an upper portion of the storage tank S. This portion of the storage tank S generally does not contain liquefied medium when the arrangement is in use.
In this known arrangement, the two transfer lines are arranged at the same side of the storage tank as shown in FIG. 1, and this can cause problems in the start-up and normal operation of the apparatus.
In operation, when the flash and boil-off gas, for example, gaseous helium, is withdrawn from the storage tank, changes in the temperature of the gaseous helium in the storage tank are transferred to apparatus downstream of the return line. When the gaseous helium is sent back to a liquefaction plant, changes in the return temperature can have a negative effect on the efficiency of the liquefaction plant. Changes in the return gas temperature occur mainly as a result of temperature stratification in the storage tank, for example, if the gas compartment sees only little flow (for example, in partial load drops, after plant start-up, in very large tanks) or when warm gas is fed into the storage tank from another source.
When the helium is continuously fed into the storage tank in the form of a two-phase mixture consisting of liquid helium and gaseous helium and at the same time continuous removal of gaseous helium from the storage tank is carried out, a bypass flow can occur and cause at least a partial flow of liquid helium back through the return transfer line.
The storage tank may need to be cooled (for example in the start-up phase of operation) by circulating cold, gaseous helium through the storage tank. Since the outlet of first transfer line 3 is in close proximity to the inlet of second transfer or return line 4, the cold gaseous helium passes only through a limited range of the tank volume and passes quickly back into the return line. In other words, the cold gaseous helium does not remain in the storage tank for any significant time, and this means it can take a long time to cool the tank down sufficiently.
Embodiments of the invention seek to provide an apparatus which overcome some or all of these problems.